Best way to add color to your 3D model?
From your supplemental comment, you seem to be creating game content for the Unity game engine, which will limit the set of possible answers, but the answer will still depend upon what kind of content you are making. Inorganic content--vehicles, metal objects, and painted surfaces, may be better colored by defining the color properties of the material. Organic content--living things (even imaginary ones)--and complex items like highly detailed structures may be best done with an image texture mapped to the surface of the model using UV unwrapping. 

If you've got some modeling experience, I'd suggest Ben Simonds' book, Blender Master Class, where he discusses a wide range of coloring techniques for various kinds of elements, organic, and inorganic.

ns